This invention relates to alkali silicatexe2x80x94polyisocyanate composites, and, more particularly, to a process for preparing alkali silicatexe2x80x94polyisocyanate composites that proceeds without catalyst separation.
Alkali silicatexe2x80x94polyisocyanate composites are frequently used in mining, tunneling, and related construction projects to consolidate and seal various types of formations, which generally comprise a void volume that is capable of receiving a curable composition that exists in a flowable state prior to its curing. The conventional method of preparing alkali silicatexe2x80x94polyisocyanate composites involves mixing a first component, which typically comprises an alkali silicate, water, and a catalyst, with a second component, which typically comprises a polyisocyanate. After the first and second components are mixed together, the reaction proceeds to form a hardened composite according to the following reaction scheme:
(1) Upon mixing, the reaction begins when some of the polyisocyanate reacts with the water to produce polyurea and gaseous carbon dioxide.
(2) Next, the in-situ formed carbon dioxide reacts instantaneously with the A2O portion of the alkali silicate to produce A2CO3xc3x97H2O (where A represents an alkali metal), while the Si2O portion of the alkali silicate reacts to form polysilicic acid.
(3) As the reaction progresses, heat is released, and the remaining polyisocyanate is trimerized.
The conventional method described above has one inherent defect. In particular, the high density of the alkali silicate tends to cause the catalyst to separate out from the alkali silicate-water-catalyst mixture and float on the top of the mixture. To minimize catalyst separation, the mixture may be mixed immediately prior to use. However, this may be difficult to do in confined areas where this mixture is often used. In addition, because catalyst separation reduces the activity of the catalyst, it is difficult to determine how much catalyst needs to be added to the reactive mixture. To counteract this problem, a surfactant may be added to the alkali silicate-water-catalyst component to keep the catalyst in solution. However, the addition of a surfactant tends to cause excessive foaming in the reaction system, thereby reducing the physical properties of the resulting composite.
While catalyst separation is the predominate problem associated with preparing alkali silicate-polyisocyanate composites, several other factors must be considered when preparing such composites. For example, the catalyst used in the preparation of the composite must not contribute to the generation of an excessive amount of carbon dioxide. The generation of an excessive amount of carbon dioxide tends to cause foaming, thereby reducing the physical properties of the resulting composite. Additionally, because alkali silicatexe2x80x94polyisocyanate composites are frequently prepared and used in confined spaces, the exotherm for the reaction preferably should not exceed about 100xc2x0 C. For the same reason, it is also preferable for the reaction to proceed without giving off an offensive odor.
Therefore, what is needed is a process for preparing alkali silicatexe2x80x94polyisocyanate composites that proceeds without catalyst separation, excessive foaming, high exotherms, or the release of an offensive odor.
The present invention provides for a process of preparing sodium silicatexe2x80x94polyisocyanate composites that proceeds without catalyst separation, excessive foaming, high exotherms, or the release of an offensive odor. To overcome deficiencies in the prior art, the present invention takes the novel approach of incorporating the catalyst into the polyisocyanate component, instead of incorporating the catalyst into the alkali silicate-water component. Incorporation of the catalyst into the polyisocyanate component prevents separation of the catalysts in the reaction mixture.
More particularly, the process of the present invention involves blending a catalyst and a polyisocyanate to form a first component, and blending an alkali silicate and water to form a second component. After blending, the first and second components are then mixed together to form a reactive mixture that reacts to form a hardened composite.
Further, the present invention also provides for sodium silicatexe2x80x94polyisocyanate composites that are prepared by blending a catalyst and a polyisocyanate to form a first component, and blending an alkali silicate and water to form a second component. After blending, the first and second components are then mixed together to form a reactive mixture that reacts to form a hardened composite.
In addition, the present invention also includes a process for consolidating and sealing various types of formations or void volumes in mining, tunneling, and related construction projects. This process involves blending a catalyst and a polyisocyanate to form a first component, and blending an alkali silicate and water to form a second component. After blending, the first and second components are then mixed together to form a reactive mixture. This reactive mixture is then introduced into a formation or void volume, and allowed to react to form a hardened composite that consolidates and/or seals the formation or void volume.